Abrasive tool making process – material – or composition – With inorganic material
Reexamination Certificate
1998-07-23
2001-10-09
Marcheschi, Michael (Department: 1755)
Abrasive tool making process, material, or composition
With inorganic material
C051S293000, C407S119000, C419S011000, C419S013000, C419S014000, C419S015000, C419S018000, C075S237000, C075S240000, C075S241000, C075S242000, C075S243000, C501S087000, C428S548000, C428S551000, C428S552000, C428S698000, C428S697000, C428S701000, C428S702000
Reexamination Certificate
active
06299658
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a tungsten carbide (hereinafter referred to as “WC”) based cemented carbide having well balanced hardness and toughness, used for cutting tools, shock resistant tools such as a bit, and for plastic working tools such as rolls and can making tools.
BACKGROUND ART
Conventionally, cemented carbide comprised of crystal grains mainly formed of WC and binder phase mainly formed of iron group metal such as Co or Ni has been used for various cutting tools and wear resistant tools as it has superior hardness, toughness and modulus of rigidity. However, along with widened application of cemented carbide recently., there has been greater need for WC based cemented carbide having higher hardness and toughness.
In order to satisfy such demand, Japanese Patent Laying-Open Nos. 2-47239, 2-138434, 2-274827 and 5-339659 propose cemented carbide in which the WC crystal grains have a plate-like shape in order to realize hardness and toughness higher than the conventional cemented carbide.
Japanese Patent Laying-Open No. 5-339659 mentioned above discloses a cemented carbide in which more than 15% of WC crystal grains in the cemented carbide are plate-like WC crystal grains having maximum dimension of 110~10 &mgr;m, which is twice or more of the minimum dimension. Japanese Patent Laying-Open No. 7-278719 or 8-199285 discloses an alloy containing plate-like WC crystal grains having the ratio of the maximum dimension with respect to the minimum dimension of 3~20 (hereinafter, this ratio will be referred to as aspect ratio: when a cemented carbide containing crystal grains mainly consisting of WC and a binder phase mainly consisting of an iron group metal contains plate-like WC crystal grains and an arbitrarily selected cross section of the cemented carbide is observed by a scanning electron microscope, the ratio of the maximum dimension with respect to the minimum dimension of an individual plate-like WC crystal grain at the arbitrary cross section).
In the proposals above, characteristics of the alloy can be improved to some extent. However, manufacturing cost has been increased, as special raw material powder or special method of manufacturing is employed. Further, the amount of generated plate-like WC crystal grains is unstable, resulting in unstable alloy characteristics.
Though toughness is improved to some extent by the generation of the plate-like WC crystal grains, strength of some plate-like WC crystal grains which coarsened too much is not necessarily higher as compared with WC crystal grains which are not coarsened, causing much variation in strength of the cemented carbide itself. Further, when WC crystal grains are coarsened, the alloy comes to have lower hardness. Therefore, development of WC based cemented carbide having more superior hardness and toughness has been desired.
SUMMARY OF THE INVENTION
The present invention was made to solve the above-described problems. An object of the present invention is to provide a cemented carbide and a cemented carbide tool having stable strength and superior hardness and toughness.
The cemented carbide in accordance with the present invention is comprised of crystal grains mainly consisting of WC and a binder phase mainly consisting of an iron group metal. Herein, the phrase “mainly consisting of . . . ” means that the largest part or portion of the stated composition consists of the stated component, according to standard dictionary definitions. In at least part of the WC crystal grains, a compound exists which is a compound of a carbide, a nitride or a carbo-nitride, of at least one component selected from the group consisting of the group IVa, Va and VIa elements or a solid solution thereof, other than WC which is the essential main component of the hard phase (in the following, “said compound” refers to the compound defined here).
The inventors made various efforts to attain the above-described object and succeeded in manufacturing a cemented carbide having stable strength and superior hardness and toughness. More specifically, the inventors of the present invention have found that by the existence of said compound in at least part of the plate-like WC crystal grains, a strain is generated in the WC crystal grains, which strain assists reinforcement of the WC crystal grains.
Japanese Patent Laying-Open No. 5-850 discloses composite hard ceramic grains in which compressive stress is generated in the WC crystal grains by dispersing a Ti compound in WC crystal grains. The powder fabricated in accordance with this method, however, does not fully exhibit its effect in liquid phase sintering as in the present invention, though it is suitable as a raw material for solid phase sintering. This may be the case that the raw material is dissolved and re-precipitated during liquid phase sintering, reducing to half the effects. The present invention allows fabrication of WC crystal grains having the above-described structure at a low cost in liquid phase sintering, without the necessity of advanced preparing a special raw material such as used in Japanese Patent Laying-Open No. 5-850. Further, according to Japanese Patent Laying-Open No. 5-850, it is necessary to disperse Ti compound of 10% to 70% by volume in order to reinforce WC crystal grains. By contrast, in the present invention, reinforcement of WC crystal grains is possible with the amount of compound dispersed to at most 10% in area ratio. The area ratio of WC crystal grains having said compound existing in the crystal grains should preferably be at least 10% and, more preferably, more than 30% of the area of all WC crystal grains.
It is particularly preferable that said compound is a carbide, a nitride or a carbo-nitride of Ti, Zr, Hf or W, or solid solution thereof. Among these, a carbide, a nitride or carbo-nitride of Zr has much effect in improving toughness and strength. The reason for this is that the compound of carbide, nitride or carbo-nitride of Ti, Zr, Hf or W or solid solution thereof is easily taken into WC crystal grains, exhibiting the effects of the present invention. The content of Ti, Zr and Hf with respect to the cemented carbide as a whole should preferably be 10 wt % at most. More preferably, the content should be at most 5 wt %. This is because too large an amount of Ti, Zr or Hf will cause a degraded sintering characteristic and a reduced strength of the cemented carbide.
It is not necessary that said compound exists only in the WC crystal grains. The compound may exist both in the WC crystal grains and the binder phase. When the grain diameter (in case of a polygon, represented by the maximum length of a diagonal, and in case of a triangle, represented by the maximum length of a side: the same applies to grain diameter of WC crystal grains) of said compound is smaller than lam, reinforcement of WC crystal grains is facilitated, remarkably improving toughness. Grain diameter of said compound not larger than 0.3 &mgr;m is particularly preferable.
When percentage by weight of the carbide, nitride or carbo-nitride of at least one component selected from Va and VIa group elements or a solid solution thereof in the cemented carbide is represented by Wa and percentage by weight of a carbide, a nitride or carbo-nitride of at least one component selected from IVa group elements or a solid solution thereof is represented by Wb, especially superior balance between toughness and hardness is exhibited if the value Wa/Wb is 0~0.2.
The reason is as follows. The compound of the carbide, nitride or carbo-nitride of a group IVa element such as Ti, Zr or Hf or a solid solution thereof is easily taken into WC crystal grains, while the compound of the carbide, nitride or carbo-nitride of at least one component selected from Va and VIa group elements or a solid solution thereof is hardly taken into WC crystal grains, and has a function of suppressing grain growth of WC crystal during sintering. Therefore, when the value of Wa/Wb is set to 0~0.2, the effects of the present invention are easily exhibited. This is the reason of numerical limitation
Ikegaya Akihiko
Moriguchi Hideki
Fasse W. F.
Fasse W. G.
Marcheschi Michael
Sumitomo Electric Industries Ltd.
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